Various types of display device such as liquid crystal display devices and plasma display devices have been utilized in recent years. Many of these display devices include an array substrate on which a plurality of signal lines and a plurality of scan lines are arranged in a matrix, with a pixel electrode being formed for each pixel that constitutes a display unit.
As array substrates, active matrix-type array substrates have been widely used in the past which are configured such that switching elements are provided for respective pixels that serve as display units in the vicinity of the intersecting portions between the aforementioned signal lines and scan lines to drive the respective pixels. Thin-film transistors (TFTs) formed by laminating a semiconductor thin film of amorphous silicon or the like, for example, a gate insulating film, an electrode, and the like are used as switching elements for such array substrates.
In order to form TFTs on an array substrate, a film formation step in which a thin film is formed on the surface of an insulating substrate, a photolithography step in which the surface of the aforementioned thin film is coated with a resist film, and the aforementioned resist film is exposed and developed to form a specified pattern using a photomask, and an etching step in which the aforementioned thin film is etched using the resist film formed in the specified pattern (this may be referred to as a resist mask in some instances) as the protective film to create the specified pattern have been repeated multiple times in the past.
In the case of the manufacture of an array substrate that uses inversely-staggered amorphous silicon TFTs, for example, a so-called five-mask process has been common in the past in which a total of five photomasks, i.e., a mask for the scan lines, a mask for the semiconductor layer (used for channel protection), a mask for the source/drain electrode, a mask for the contact holes (for passivation), and a mask for the pixel electrodes, was normally used, repeating the photolithography step five times for the respective photomasks. When an array substrate is to be manufactured, reducing the mask process in the photolithography steps is an extremely important target in order to reduce the manufacturing cost by effects obtained by reducing the cost of the masks, shortening the manufacturing steps, cutting down the man hours, and the like and to increase the yield.
Various four-mask processes in which an array substrate is manufactured with one photomask being reduced have been proposed in the manufacture of array substrates. As one of four-mask processes, a method for using a so-called reflow process in which one of the photolithography steps is omitted by causing the resist film to swell/be softened in a vapor atmosphere of an organic solvent and changing the shape of the resist film is publicly known (for example, see Patent Document 1). By using the aforementioned reflow process, it is possible to omit the photolithography step that uses a mask for the semiconductor layer (for channel protection).